1. Field of the Invention
This invention relates to a radiation resistant ring laser gyro (RLG), particularly an RLG which will continue to operate when exposed to high energy radiation.
2. The Prior Art
RLGs are known in the prior art and are employed, e.g., for measuring rotation in inertial space about the input axis. By way of further background, an RLG is a highly accurate optical rotation sensor using counter propagating light beams to sense rotation as a rate integrating gyro. When combined with 3 accelerometers, a three axis RLG yields an inertial sensor assembly which may be directly applied to navigation, guidance and control systems for vehicles, including satellites and other space vehicles.
An example of the above prior art systems and photo detector is shown in FIGS. 1 and 2 hereof and further discussed below.
A problem with such RLG (opto-electronics) systems is that they can be put out of commission partly or wholly, by a high energy event including a nuclear event. That is, prior art RLG systems suffer blindness or loss of output data during and after exposure to high energy radiation.
Attempts have been made to harden the RLG systems including, the use of shielding, synthetic sapphire optical elements and special high cost radiation-hard, crystalline photodiodes have been fixed to the RLG output prism using epoxy. There are few variants of these methods and the RLG today is still "blinded" or charge saturated during a high energy event. This causes data loss which deleteriously impacts a tactical and strategic mission effectiveness of vehicles including space vehicles, weapons and delivery vehicles therefor incorporating the RLG into inertial navigation systems. Such RLG blindenss today remains a problem of serious magnitude.
The shortcomings of the prior art RLGS are ostensibly concentrated in the areas of volumetric factors and charge-transport physics of the photoconductors used to detect RLG fringe patterns. The volumetric factor is that the smaller the volume of the optoelectronics in a high energy environment, the lower the radiation sensitivity of the device under consideration. The photodiodes glued to the output face of the prism, constitute a relatively large volume device as do their associated cathode and anode leads. The photodiode employed is generally crystalline in nature and as such, saturates quickly in a high energy radiation event, due to the high injected charge density and excellent carrier mobility of its crystalline materials. This saturation effect causes data loss since the injected charge fills the transport channel (of such photodiode) with large quantities of electron-hole pairs and the fringes of the RLG (Sagnac interferometer) are not detected during the event.
The above initial output data loss or RLG blindness is known as the "prompt effect" to prior art RLG photodiodes during a high energy radiation (including nuclear) event. That is, during such high energy pulsed event, there is a charge build-up in the RLG detector material, which quickly saturates or whites out and most output data collected during such event is lost. Again, the larger the volume of a detector device, the greater such "prompt effect" in disabling same.
There is also a more durable after-effect to such prior art RLG detector devices after the pulse high energy radiation has passed, that impairs the subsequent operation thereof known as the "delayed effect, including a photodarkening of such photodiodes or other prior art detector devices. That is, the burst of high energy particles can impart enough energy to the lattice of the detector material to cause atomic displacement thereof, which can result in photo darkening the material or color center creation therein, which can cause electronic defects in the detector material transport channel. Such photo darkening effect means a carrier (data) that is generated by an RLG output prism, will be stopped or trapped in the transport channel of the detector and will not be transmitted to the rest of the data collection circuit or will be transmitted in a distorted manner. The more the photo darkening effect in such detector, the more radiation-induced defects therein, the poorer will the RLG detector perform (e.g., in a navigation system) well after the high energy pulse radiation has passed.
Accordingly, there is a need and market for an RLG system, including a photodetector that significantly overcomes the above prior art shortcomings.
There has now been discovered an RLG system, including one or more RLG detectors, that address and overcome 1) the above "prompt effect" during a high energy pulsed event and 2) the above "delayed effect", so as to continue with data output during and after such high energy event. Further, such RLG output detectors of the present invention can be manufactured at greatly reduced cost over the lesser performing detectors of the prior art.